Internationalized Resource Identifiers (IRIs)World Wide Web Consortium5322 EndoFujisawaKanagawa252-8520Japan+81 466 49 1170+81 466 49 1171mailto:duerst@w3.orghttp://www.w3.org/People/D%C3%BCrst/ (Note: This is the percent-encoded form of an IRI.)Microsoft CorporationOne Microsoft WayRedmondWA98052U.S.A.+1 425 882-8080mailto:michelsu@microsoft.comhttp://www.suignard.comIRIInternationalized Resource IdentifierUTF-8URIURLIDNThis document defines a new protocol element, the Internationalized
Resource Identifier (IRI), as a complement to the Uniform Resource Identifier (URI).
An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646).
A mapping from IRIs to URIs is defined, which means that IRIs can be used instead
of URIs where appropriate to identify resources.The approach of defining a new protocol element was chosen, instead
of extending or
changing the definition of URIs, to allow a clear distinction and to
avoid incompatibilities with existing software. Guidelines for the
use and deployment of IRIs in various protocols, formats, and software
components that now deal with URIs are provided.A Uniform Resource Identifier (URI) is defined in
as a sequence of characters
chosen from a limited subset of the repertoire of US-ASCII
characters.The characters in URIs are frequently used for representing words of
natural languages. Such usage has many advantages: such URIs are easier to
memorize, easier to interpret, easier to transcribe, easier to create,
and easier to guess. For most languages other than English, however,
the natural script uses characters other than A-Z. For many people,
handling Latin characters is as difficult as handling the characters
of other scripts is for people who use only the Latin alphabet. Many
languages with non-Latin scripts have transcriptions to Latin
letters. Such transcriptions are now often used in URIs, but they
introduce additional ambiguities.The infrastructure for the appropriate handling of characters from
local scripts is now widely deployed in local versions of operating
system and application software. Software that can handle a wide
variety of scripts and languages at the same time is increasingly
widespread. Also, there are increasing numbers of protocols and
formats that can carry a wide range of characters.This document defines a new protocol element, called Internationalized
Resource Identifier (IRI), by extending the syntax of URIs
to a much wider repertoire of characters. It also defines "internationalized"
versions corresponding to other constructs from ,
such as URI references. The syntax of IRIs is defined in ,
and the relationship between IRIs and URIs in .
Using characters outside of A-Z in IRIs brings with it some
difficulties. discusses the special case of
bidirectional IRIs, various forms of
equivalence between IRIs, and the use of
IRIs in different situations. gives
additional informative guidelines, and
security considerations.IRIs are designed to be compatible with recommendations for new URI
schemes . The compatibility is provided by specifying
a well defined and deterministic mapping from the IRI character sequence to
the functionally equivalent URI character sequence. Practical use of IRIs
(or IRI references) in place of URIs (or URI references) depends on the
following conditions being met:
The protocol or format element where IRIs are used should be explicitly
designated to be able to carry IRIs. That is, the intent is not to
introduce IRIs into contexts that are not defined to accept them.
For example, XML schema has an explicit type "anyURI"
that includes IRIs and IRI references. Therefore, IRIs and IRI references
can be in attributes and elements of type "anyURI".
On the other hand, in the HTTP protocol , the Request URI is
defined as an URI, which means that direct use of IRIs is not allowed
in HTTP requests.The protocol or format carrying the IRIs should have a
mechanism to represent the wide range of characters used in IRIs, either
natively or by some protocol- or format-specific escaping mechanism
(for example numeric character references in ).The URI corresponding to the IRI in question has to
encode original characters into octets using UTF-8. For new URI schemes,
this is recommended in . It can apply to a whole
scheme (e.g. IMAP URLs and POP
URLs , or the URN syntax ).
It can apply to a specific part of a URI, such as the fragment identifier
(e.g. ). It can apply to a specific URI or part(s)
thereof. For details, please see .The following definitions are used in this document; they follow the
terms in , and
:A member of a set of elements used for the organization,
control, or representation of data. For example, "LATIN CAPITAL LETTER A" names
a character.An ordered sequence of eight bits considered as a unitA set of characters (in the mathematical sense)A sequence (one after another) of charactersA sequence (one after another) of octetsA method of representing a sequence of
characters as a sequence of octets (maybe with variants). A method of
(unambiguously) converting a sequence of octets into a sequence of characters.The name of a parameter or attribute used
to identify a character encoding.Universal Character Set; the coded character set defined by
ISO/IEC 10646 and
the Unicode Standard .The term "IRI reference" denotes the common usage
of an Internationalized Resource Identifier. An IRI reference may be absolute or relative.
However, the "IRI" that results from such a reference only includes absolute IRIs;
any relative IRI references are resolved to their absolute form.
Note that in , URIs did not include fragment identifiers,
but in , fragment identifiers are part of URIs.Human text (paragraphs, sentences, phrases)
with syntax according to orthographic conventions of a natural language,
as opposed to syntax defined for ease of processing by machines (markup,
programming languages,...).Any portion of a message which affects
processing of that message by the protocol in question.Presentation form corresponding to a
protocol element, for example using a wider range of characters.With respect to URIs and IRIs, the
word 'create' is used for the initial creation. This may be the
initial creation of a resource with a certain identifier, or the initial
exposition of a resource under a particular identifier.With respect to URIs and IRIs,
the word 'generate' is used when the IRI is generated by derivation
from other information.RFCs and Internet Drafts currently do not allow any characters outside
the US-ASCII repertoire. Therefore, this document uses various special
notations to denote such characters in examples.In text, characters outside US-ASCII are sometimes referenced by
using a prefix of 'U+', followed by four to six hexadecimal digits.To represent characters outside US-ASCII in examples, this document
uses two notations called 'XML Notation' and 'Bidi Notation'.XML Notation uses leading '&#x', trailing ';', and the
hexadecimal number of the character in the UCS in between. Example:
&#x44F; stands for CYRILLIC CAPITAL LETTER YA. In this notation, an
actual '&' is denoted by '&amp;'.Bidi Notation is used for bidirectional examples: lower case
letters stand for Latin letters or other letters that are written left-to-right,
whereas upper case letters represent Arabic or Hebrew letters that are
written right-to-left.To denote actual octets in examples (as opposed to percent-encoded octets),
the two hex digits denoting the octet are enclosed in "<" and ">".
For example, the octet often denoted as 0xc9 is denoted here as <c9>. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
.This section defines the syntax of Internationalized Resource
Identifiers (IRIs).As with URIs, an IRI is defined as a sequence of characters, not as
a sequence of octets. This
definition accommodates the fact that IRIs may be written on paper or
read over the radio as well as being stored or transmitted digitally.
The same IRI may be represented as different sequences of octets
in different protocols or documents if these protocols or documents
use different character encodings (and/or transfer encodings).
Using the same character encoding
as the containing protocol or document assures that the characters in
the IRI can be handled (searched, converted, displayed,...) in the
same way as the rest of the protocol or document.IRIs are defined similarly to URIs in ,
but the class of unreserved characters is extended by
adding the characters of the UCS (Universal Character Set,
) beyond U+007F, subject to the limitations given
in the syntax rules below and in .Otherwise, the syntax and use of components and reserved characters is
the same as that in . All the operations defined in
, such as the resolution of relative references, can be
applied to IRIs by IRI-processing software in exactly the same way as this
is done to URIs by URI-processing software.Characters outside the US-ASCII repertoire are not reserved and therefore
MUST NOT be used for syntactical
purposes such as to delimit components in newly defined schemes. As an
example, it is not allowed to use U+00A2, CENT SIGN, as a delimiter in IRIs,
because it is in the 'iunreserved' category, in the same way as it is
not possible to use '-' as a delimiter, because it is in the
'unreserved' category in URIs.While it might be possible to define IRI references and IRIs merely by
their transformation to URI references and URIs, they can also be
accepted and processed directly. Therefore, an ABNF definition for
IRI references (which are the most general concept and the start of the
grammar) and IRIs is given here. The syntax of this ABNF is described in
. Character numbers are taken from the UCS,
without implying any actual binary encoding. Terminals in the ABNF
are characters, not bytes.The following grammar closely follows the URI grammar in
, except that the range of unreserved characters
is expanded to include UCS characters, with the restriction that private
UCS characters can occur only in query parts and not elsewhere. The
grammar is split into two parts, rules that differ from
because of the above-mentioned expansion, and rules that are the same
as in . For rules that are different than in
, the names of the non-terminals have been
changed as follows: If the non-terminal contains 'URI', this has
been changed to 'IRI'. Otherwise, an 'i' has been prefixed.The following rules are different from :
IRI = scheme ":" ihier-part [ "?" iquery ]
[ "#" ifragment ]
ihier-part = "//" iauthority ipath-abempty
/ ipath-absolute
/ ipath-rootless
/ ipath-empty
IRI-reference = IRI / irelative-ref
absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
irelative-ref = irelative-part [ "?" iquery ] [ "#" ifragment ]
irelative-part = "//" iauthority ipath-abempty
/ ipath-absolute
/ ipath-noscheme
/ ipath-empty
iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo = *( iunreserved / pct-encoded / sub-delims / ":" )
ihost = IP-literal / IPv4address / ireg-name
ireg-name = *( iunreserved / pct-encoded / sub-delims )
ipath = ipath-abempty ; begins with "/" or is empty
/ ipath-absolute ; begins with "/" but not "//"
/ ipath-noscheme ; begins with a non-colon segment
/ ipath-rootless ; begins with a segment
/ ipath-empty ; zero characters
ipath-abempty = *( "/" isegment )
ipath-absolute = "/" [ isegment-nz *( "/" isegment ) ]
ipath-noscheme = isegment-nz-nc *( "/" isegment )
ipath-rootless = isegment-nz *( "/" isegment )
ipath-empty = 0<ipchar>
isegment = *ipchar
isegment-nz = 1*ipchar
isegment-nz-nc = 1*( iunreserved / pct-encoded / sub-delims
/ "@" )
; non-zero-length segment without any colon ":"
ipchar = iunreserved / pct-encoded / sub-delims / ":"
/ "@"
iquery = *( ipchar / iprivate / "/" / "?" )
ifragment = *( ipchar / "/" / "?" )
iunreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" / ucschar
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
Some productions are ambiguous. The "first-match-wins" (a.k.a. "greedy")
algorithm applies. For details, see .The following are the same as in :
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
port = *DIGIT
IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims
/ ":" )
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet
"." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
This syntax does not support IPv6 scoped addressing zone identifiers.IRIs are meant to replace URIs in identifying resources for protocols,
formats and software components which use a UCS-based character repertoire.
These protocols and components may never need to use URIs directly,
especially when the resource identifier is used simply for identification
purposes. However, when the resource identifier is used for resource retrieval,
it is in many cases necessary to determine the associated URI because most
retrieval mechanisms currently only are defined for URIs. In this case,
IRIs can serve as presentation elements for URI protocol elements.
An example would be an address bar in a Web user agent.
(Additional rationale is given in .)This section defines how to map an IRI to a URI. Everything in
this section applies also to IRI references and URI references, as
well as components thereof (for example fragment identifiers).This mapping has two purposes:Many URI schemes and components define additional
syntactical restrictions not captured in .
Scheme-specific restrictions are applied to IRIs by converting
IRIs to URIs and checking the URIs against the scheme-specific
restrictions.URIs identify resources in various ways.
IRIs also identify resources. When the IRI is used solely for identification
purposes, it is not necessary to map the IRI to a URI (see ).
However, when an IRI is used for resource retrieval, the resource that the IRI
locates is the same as the one located by the URI obtained after
converting the IRI according to the procedure defined here.
This means that there is no need to define resolution separately on the IRI level.Applications MUST map IRIs to URIs using the following two steps.This step generates a UCS character sequence from
the original IRI format.
This step has three variants, depending on the form of the input.
If the IRI is written on paper or read out loud, or
otherwise represented as a sequence of characters independent of any
character encoding:
Represent the IRI as a sequence of characters from the UCS
normalized according to Normalization Form C (NFC, ).
If the IRI is in some digital representation
(e.g. an octet stream) in some known non-Unicode character encoding:
Convert the IRI to a sequence of characters from the UCS
normalized according to NFC.
If the IRI is in an Unicode-based character encoding (for example UTF-8 or UTF-16):
Do not normalize (see for details). Apply Step 2 directly to the encoded Unicode character sequence.
For each character in 'ucschar' or 'iprivate', apply
Steps 2.1 through 2.3 below.
Convert the character to a sequence of one or more octets
using UTF-8 .Convert each octet to %HH, where HH is the hexadecimal
notation of the octet value. Note that this is identical to the
percent-encoding mechanism in Section 2.1 of .
To reduce variability, the hexadecimal notation SHOULD use upper
case letters.Replace the original character with the resulting character
sequence (i.e., a sequence of %HH triplets).The above mapping from IRIs to URIs produces URIs fully conforming to
. The mapping is also an identity transformation for URIs
and is idempotent -- applying the mapping a second time will not change
anything. Every URI is by definition an IRI.Infrastructure accepting IRIs MAY convert the ireg-name
component of an IRI as follows (before Step 2 above) for schemes
that are known to use domain names in ireg-name, but where the
scheme definition does not allow percent-encoding for ireg-name:
Replace the ireg-name part of the IRI by the part converted using
the ToASCII operation specified in Section 4.1 of
on each dot-separated label, and using U+002E (FULL STOP) as a label
separator, with the flag UseSTD3ASCIIRules set to TRUE and the flag
AllowUnassigned set to FALSE for creating IRIs and set to TRUE otherwise.
The ToASCII operation may fail, but this would mean that the IRI cannot
be resolved. This conversion SHOULD be used when the goal is to
maximize interoperability with legacy URI resolvers.
For example, the IRI
http://r&#xE9;sum&#xE9;.example.org may be converted to
http://xn--rsum-bpad.example.org instead of
http://r%C3%A9sum%C3%A9.example.org.An IRI with a scheme that is known to use domain names in ireg-name,
but where the scheme definition does not allow percent-encoding for ireg-name,
meets scheme-specific restrictions if either the straightforward
conversion or the conversion using the ToASCII operation on ireg-name
result in an URI that meets the scheme-specific restrictions.
Such an IRI
resolves to the URI obtained after converting the IRI including using
the ToASCII operation on ireg-name. Implementations do not need to
do this conversion as long as they produce the same result.The difference between Variants B and C
in Step 1 (Variant B using normalization with NFC while
Variant C not using any normalization) is to account for the
fact that in many non-Unicode character encodings, some text
cannot be represented directly. For example, Vietnam is
natively written "Vi&#x1EC7;t Nam" (containing a LATIN SMALL
LETTER E WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct
transcoding from the windows-1258 character encoding leads to
"Vi&#xEA;&#x323;t Nam" (containing a LATIN SMALL LETTER E WITH CIRCUMFLEX
followed by a COMBINING DOT BELOW), whereas direct transcoding
of other 8-bit encodings of Vietnamese may lead to other
representations.The uniform treatment of the whole IRI in Step 2
above is important to not make processing dependent on URI scheme.
See for an in-depth discussion.In practice, the difference above will not be
noticed if mapping from IRI to URI and resolution is tightly
integrated (e.g. carried out in the same user agent). But conversion
using may be able to better deal with
backwards compatibility issues in case mapping and resolution
are separated, as in the case of using an HTTP proxy.Internationalized Domain Names may be contained in parts
of an IRI other than the ireg-name part.
It is the responsibility of scheme-specific
implementations (if the Internationalized Domain Name is part of the
scheme syntax) or of server-side implementations (if the Internationalized
Domain Name is part of 'iquery') to apply the necessary conversions
at the appropriate point. Example: Trying to validate the Web page at
http://r&#xE9;sum&#xE9;.example.org would lead to an IRI of
http://validator.w3.org/check?uri=http%3A%2F%2Fr&#xE9;sum&#xE9;.example.org,
which would convert to a URI of
http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.example.org.
The server side implementation would be responsible to do the necessary
conversions in order to be able to retrieve the Web page.Infrastructure accepting IRIs MAY also deal with the printable characters
in US-ASCII that are not allowed in URIs, namely "<", ">", '"', Space,
"{", "}", "|", "\", "^", and "`", in Step 2 above. If such characters are
found but are not converted, then the conversion SHOULD fail.
Please note that the number sign ("#"), the percent sign ("%"), and the
square bracket characters ("[", "]")
are not part of the above list, and MUST NOT be converted.
Protocols and formats that have used earlier definitions of IRIs
including these characters MAY require percent-encoding of these characters
as a preprocessing step to extract the actual IRI from a given field.
Such preprocessing MAY also be used by applications allowing the user
to enter an IRI.In this process (in Step 2.3), characters allowed in URI
references as well as existing percent-encoded sequences are not encoded further.
(This mapping is similar to, but different from, the encoding applied
when including arbitrary content into some part of a URI.)
For example, an IRI of
http://www.example.org/red%09ros&#xE9;#red
(in XML notation) is converted to
http://www.example.org/red%09ros%C3%A9#red, not to
something like
http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.Some older software transcoding to UTF-8 may produce
illegal output for some input, in particular for characters outside
the BMP (Basic Multilingual Plane). As an example, for the following
IRI with non-BMP characters (in XML Notation):
http://example.com/&#x10300;&#x10301;&#x10302;
(the first three letters of the Old Italic alphabet)
the correct conversion to a URI is:
http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82In some situations, it may be desirable to try to convert a URI into an
equivalent IRI. This section gives a procedure to do such a conversion.
The conversion described in this section
will always result in an IRI which maps back to the URI that was used
as an input for the conversion (except for potential case differences
in percent-encoding and for potential percent-encoded unreserved characters).
However, the IRI resulting from this conversion
may not be exactly the same as the original IRI (if there ever was one).URI to IRI conversion removes percent-encodings, but not all
percent-encodings can be eliminated. There are several reasons for this:Some percent-encodings are necessary to distinguish
percent-encoded and unencoded uses of reserved characters.Some percent-encodings cannot be interpreted as sequences
of UTF-8 octets.
(Note: The octet patterns of UTF-8 are highly regular.
Therefore, there is a very high probability, but no guarantee,
that percent-encodings that can be interpreted as sequences of UTF-8
octets actually originated from UTF-8. For a detailed discussion,
see .)The conversion may result in a character that is not
appropriate in an IRI. See , ,
and for further details.Conversion from a URI to an IRI is done using the following steps
(or any other algorithm that produces the same result):
Represent the URI as a sequence of octets in US-ASCII.Convert all percent-encodings (% followed by two hexadecimal
digits) except those corresponding to '%', characters in 'reserved',
and characters in US-ASCII not allowed in URIs,
to the corresponding octets.Re-percent-encode any octet produced in Step 2 that is not part of a
strictly legal UTF-8 octet sequence.Re-percent-encode all octets produced in Step 3 that in UTF-8 represent
characters that are not appropriate according to ,
, and .Interpret the resulting octet sequence as a sequence of characters
encoded in UTF-8.
This procedure will convert as many percent-encoded characters as
possible to characters in an IRI. Because there are some choices
when applying Step 4 (see ), results may vary.Conversions from URIs to IRIs MUST NOT use any other character encoding than
UTF-8 in Steps 3 and 4 above, even if it might be possible from context
to guess that another character encoding than UTF-8 was used in the URI.
As an example, the URI http://www.example.org/r%E9sum%E9.html might with
some guessing be interpreted to contain two e-acute characters encoded as
iso-8859-1. It must not be converted to an IRI containing these e-acute
characters. Otherwise, the IRI will in the future be mapped to
http://www.example.org/r%C3%A9sum%C3%A9.html, which is a different URI
than http://www.example.org/r%E9sum%E9.html.This section shows various examples of converting URIs to IRIs.
Each example shows the
result after applying each of the Steps 1 to 5. XML Notation is
used for the final result.
The following example contains the sequence '%C3%BC', which is a strictly
legal UTF-8 sequence, and which is converted into the actual character
U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known as u-umlaut).
http://www.example.org/D%C3%BCrsthttp://www.example.org/D<c3><bc>rsthttp://www.example.org/D<c3><bc>rsthttp://www.example.org/D<c3><bc>rsthttp://www.example.org/D&#xFC;rstThe following example contains the sequence '%FC', which might represent U+00FC
LATIN SMALL LETTER U WITH DIAERESIS in theiso-8859-1 character encoding.
(It might represent other characters in other character encodings. For example, the octet
<fc> in iso-8859-5 represents U+045C CYRILLIC SMALL LETTER KJE.)
Because <fc> is not part of a
strictly legal UTF-8 sequence, it is re-percent-encoded in Step 3.
http://www.example.org/D%FCrsthttp://www.example.org/D<fc>rsthttp://www.example.org/D%FCrsthttp://www.example.org/D%FCrsthttp://www.example.org/D%FCrstThe following example contains '%e2%80%ae', which is the percent-encodedUTF-8
character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.
forbids the direct use of this character in an IRI. Therefore, the
corresponding octets are re-percent-encoded in Step 4. This example shows
that the case (upper or lower) of letters used in percent-encodes may not be preserved.
The example also contains a punycode-encoded domain name label (xn--99zt52a),
which is not converted.
http://xn--99zt52a.example.org/%e2%80%aehttp://xn--99zt52a.example.org/<e2><80><ae>http://xn--99zt52a.example.org/<e2><80><ae>http://xn--99zt52a.example.org/%E2%80%AEhttp://xn--99zt52a.example.org/%E2%80%AEImplementations with scheme-specific knowledge MAY convert
punycode-encoded domain name labels to the corresponding characters using the
ToUnicode procedure. Thus, for the example above, the label xn--99zt52a may
be converted to U+7D0D U+8C46 (Japanese Natto), leading to the overall
IRI ofhttp://&#x7D0D;&#x8C46;.example.org/%E2%80%AESome UCS characters, such as those used in the Arabic and Hebrew
script, have an inherent right-to-left (rtl) writing direction. IRIs
containing such characters (called bidirectional IRIs or Bidi IRIs)
require additional attention because of the non-trivial relation
between logical representation (used for digital representation as
well as when reading/spelling) and visual representation (used for
display/printing).Because of the complex interaction between the logical representation,
the visual representation, and the syntax of a Bidi IRI, a balance is
needed between various requirements.
The main requirements are:
user-predictable conversion between visual and
logical representation;the ability to include a wide range of characters
in various parts of the IRI;minor or no changes or restrictions for
implementations.When stored or transmitted in digital representation,
bidirectional IRIs MUST be in full logical order, and
MUST conform to the IRI syntax rules (which includes the rules
relevant to their scheme). This assures that bidirectional
IRIs can be processed in the same way as other IRIs.When rendered, bidirectional IRIs MUST be rendered using the
Unicode Bidirectional Algorithm , .
Bidirectional IRIs MUST be rendered in the same way as they would
be rendered if they were in an left-to-right embedding, i.e. as if
they were preceded by U+202A, LEFT-TO-RIGHT EMBEDDING (LRE),
and followed by U+202C, POP DIRECTIONAL FORMATTING (PDF).
Setting the embedding direction can also be done in a
higher-level protocol (e.g. the dir='ltr' attribute in HTML).There is no requirement to actually use the above embedding
if the display is still the same without the embedding. For
example, a bidirectional IRI in a text with left-to-right base
directionality (such as used for English or Cyrillic) that is
preceded and followed by whitespace and strong left-to-right
characters does not need an embedding.
Also, a bidirectional relative IRI reference that only contains strong
right-to-left characters and weak characters and that starts and
ends with a strong rigth-to-left character and appears in a text with
right-to-left base directionality (such as used for Arabic or Hebrew)
and is preceded and followed by whitespace and strong characters
does not need an embedding.In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM)
may be sufficient to force the correct display behavior.
However, the details of the Unicode Bidirectional algorithm are
not always easy to understand. Implementers are strongly advised
to err on the side of caution and to use embedding in all cases
where they are not completely sure that the display behavior
is unaffected without the embedding.The Unicode Bidirectional Algorithm (,
Section 4.3) permits higher-level protocols to influence bidirectional
rendering. Such changes by higher-level protocols MUST NOT be used
if they change the rendering of IRIs.The bidirectional formatting characters that may be used before or
after the IRI to assure correct display are themselves not part of the IRI.
IRIs MUST NOT contain bidirectional formatting characters (LRM,
RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering
of the IRI, but do not themselves appear visually. It would therefore
not be possible to correctly input an IRI with such characters.The Unicode Bidirectional Algorithm is designed mainly for running text.
To make sure that it does not affect the rendering of bidirectional IRIs
too much, some restrictions on bidirectional IRIs are necessary. These
restrictions are given in terms of delimiters (structural characters,
mostly punctuation such as '@', '.', ':','/') and components (usually
consisting mostly of letters and digits).The following syntax rules from correspond to components
for the purpose of Bidi behavior: iuserinfo, ireg-name, isegment, isegment-nz,
isegment-nz-nc, ireg-name, iquery, and ifragment.
Specifications that define the syntax of any of the above components
MAY divide them further and define smaller parts to be components according
to this document. As an example, the restrictions of on
bidirectional domain names correspond to treating each label of a domain
name as a component for those schemes where ireg-name is a domain name.
Even where the components are not defined formally, it may be helpful to
think about some syntax in terms of components and to apply the relevant restrictions.
For example, for the usual name/value syntax in query parts, it is convenient to
treat each name and each value as a component. As another example, the
extensions in a resource name can be treated as separate components.For each component, the following restrictions apply:A component SHOULD NOT use both right-to-left and left-to-right
characters.A component using right-to-left characters SHOULD start and end
with right-to-left characters.The above restrictions are given as shoulds, rather than as musts.
For IRIs that are never presented visually, they are not relevant.
However, for IRIs in general, they are very important to insure
consistent conversion between visual presentation and logical
representation, in both directions.
In some components, the above restrictions may actually be
strictly enforced.
For example, requires that these restrictions apply
to the labels of a host name for those schemes where ireg-name is a host name.
In some other components,
for example path components, following these restrictions may not be too difficult.
For other components, such as parts of the query part, it may be very difficult
to enforce the restrictions, because the values of query parameters may be
arbitrary character sequences.If the above restrictions cannot be satisfied otherwise, the affected
component can always be mapped to URI notation as described in
. Please note that the whole component needs
to be mapped (see also Example 9 below).
Bidi input methods MUST generate Bidi IRIs in logical
order while rendering them according to .
During input, rendering SHOULD be updated after every new character
that is input to avoid end user confusion.This section gives examples of bidirectional IRIs, in Bidi Notation.
It shows legal IRIs with the relationship between logical and
visual representation, and explains how certain phenomena in
this relationship may look strange to somebody not familiar
with bidirectional behavior, but familiar to users of Arabic
and Hebrew. It also shows what happens if the
restrictions given in are not
followed. The examples below can be seen at ,
in Arabic, Hebrew, and Bidi Notation variants.To read the bidi text in the examples, read the visual
representation from left to right until you encounter a block of
rtl text. Read the rtl block (including slashes and other special
characters) from right to left, then continue at the next unread ltr
character.Example 1: A single component with rtl
characters is inverted:
logical representation: http://ab.CDEFGH.ij/kl/mn/op.html
visual representation: http://ab.HGFEDC.ij/kl/mn/op.html
Components can be read one-by-one, and each component can be
read in its natural direction.Example 2: More than one consecutive component with rtl
characters is inverted as a whole:
logical representation: http://ab.CDE.FGH/ij/kl/mn/op.html
visual representation: http://ab.HGF.EDC/ij/kl/mn/op.html
A sequence of rtl components is read rtl, in the same way
as a sequence of rtl words is read rtl in a bidi text.Example 3: All components of an IRI (except for the scheme) are rtl.
All rtl components are inverted overall:
logical representation: http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV
visual representation: http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA
The whole IRI (except the scheme) is read rtl. Delimiters
between rtl components stay between the respective components;
delimiters between ltr and rtl components don't move.Example 4: Several sequences of rtl components are each
inverted on their own:
logical representation: http://AB.CD.ef/gh/IJ/KL.html
visual representation: http://DC.BA.ef/gh/LK/JI.html
Each sequence of rtl components is read rtl, in the same way
as each sequence of rtl words in an ltr text is read rtl.Example 5: Example 2, applied to components of different
kinds:
logical representation: http://ab.cd.EF/GH/ij/kl.html
visual representation: http://ab.cd.HG/FE/ij/kl.html
The inversion of the domain name label and the path component
may be unexpected, but is consistent with other bidi behavior.
For reassurance that the domain component really is "ab.cd.EF",
it may be helpful to read aloud the visual representation following
the bidi algorithm. After "http://ab.cd." one reads the RTL block
"E-F-slash-G-H", which corresponds to the logical representation.
Example 6: Same as example 5, with more rtl components:
logical representation: http://ab.CD.EF/GH/IJ/kl.html
visual representation: http://ab.JI/HG/FE.DC/kl.html
The inversion of the domain name labels and the path components
may be easier to identify because the delimiters also move.Example 7: A single rtl component with included digits:
logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html
visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html
Numbers are written ltr in all cases, but are treated as
an additional embedding inside a run of rtl characters. This
is completely consistent with usual bidirectional text.Example 8 (not allowed): Numbers at the start or end of a rtl component:
logical representation: http://ab.cd.ef/GH1/2IJ/KL.html
visual representation: http://ab.cd.ef/LK/JI1/2HG.html
The sequence '1/2' is interpreted by the bidi algorithm
as a fraction, fragmenting the components and leading to
confusion. There are other characters that are interpreted
in a special way close to numbers, in particular '+', '-',
'#', '$', '%', ',', '.', and ':'.Example 9 (not allowed): The numbers in the previous
example are percent-encoded:
logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
visual representation (Hebrew): http://ab.cd.ef/%31HG/LK/JI%32.html
visual representation (Arabic): http://ab.cd.ef/31%HG/%LK/JI32.html
Depending on whether the upper-case letters represent Arabic or
Hebrew, the visual representation is different.Example 10 (allowed, but not recommended):
logical representation: http://ab.CDEFGH.123/kl/mn/op.html
visual representation: http://ab.123.HGFEDC/kl/mn/op.html
Components consisting
of only numbers are allowed (it would be rather difficult to prohibit
them), but may interact with adjacent RTL components in ways that are
not easy to predict.Note: The structure and much of the material for this
section is taken from section 6 of ; the
differences are due to the specifics of IRIs.One of the most common operations on IRIs is simple comparison: determining
if two IRIs are equivalent without using the IRIs or the mapped URIs to access
their respective resource(s). A comparison is performed every time a response
cache is accessed, a browser checks its history to color a link, or an XML
parser processes tags within a namespace. Extensive normalization prior to
comparison of IRIs may be used by spiders and indexing engines to prune a
search space or reduce duplication of request actions and
response storage.IRI comparison is performed in respect to some particular purpose, and
implementations with differing purposes will often be subject to differing
design trade-offs in regards to how much effort should be spent in
reducing aliased identifiers. This section describes a variety of methods
that may be used to compare IRIs, the trade-offs between them, and the types
of applications that might use them.Since IRIs exist to identify resources, presumably they should be considered
equivalent when they identify the same resource. However, such a definition of
equivalence is not of much practical use, since there is no
way for an implementation to compare two resources that are not under its own
control. For this reason, determination of equivalence or difference of IRIs
is based on string comparison, perhaps augmented by reference to additional
rules provided by URI scheme definitions.
We use the terms "different" and
"equivalent" to describe the possible outcomes of such comparisons, but there
are many applicationdependent versions of equivalence.Even though it is possible to determine that two IRIs are equivalent,
IRI comparison is not sufficient to determine if two IRIs identify different
resources. For example, an owner of two different domain names
could decide to serve the same resource from both, resulting in two
different IRIs. Therefore, comparison methods are designed to minimize
false negatives while strictly avoiding false positives.In testing for equivalence, applications should not directly compare
relative references; the references should be converted to their respective
target IRIs before comparison. When IRIs are being compared for
the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, fragment components (if any)
should be excluded from the comparison.Applications using IRIs as identity tokens with no relationship to a
protocol MUST use the Simple String Comparison (see ).
All other applications MUST select one of the comparison practices from
the Comparison Ladder (see , or, after IRI-to-URI conversion,
select one of the comparison practices from the URI comparison
ladder , Section 6.2.Any kind of IRI comparison REQUIRES that all escapings or encodings in the protocol or format that carries an IRI are resolved. This is usually done when parsing the protocol or format. Examples of such escapings or encodings are entities and numeric character references in and . As an example, http://example.org/ros&eacute; (in HTML), http://example.org/ros&#233; (in HTML or XML), and http://example.org/ros&#xE9; (in HTML or XML) all get resolved into what is denoted in this document (see ) as http://example.org/ros&#xE9; (the "&#xE9;" here standing for the actual e-acute character, to compensate for the fact that this document cannot contain non-ASCII characters).Similar considerations apply to encodings such as Transfer Codings in HTTP (see ) and Content Transfer Encodings in MIME, although in these cases, the encoding is not based on characters, but on octets, and additional care is required to make sure that characters, and not just arbitrary octets, are compared (see ).A variety of methods are used in practice to test
IRI equivalence. These methods fall into a range,
distinguished by the amount of processing required
and the degree to which the probability of false
negatives is reduced. As noted above, false negatives
cannot be eliminated. In practice, their probability can
be reduced, but this reduction requires more processing
and is not cost-effective for all applications.If this range of comparison practices is considered
as a ladder, the following discussion will climb the ladder,
starting with those practices that are cheap but have
a relatively higher chance of producing false negatives,
and proceeding to those that have higher computational
cost and lower risk of false negatives.If two IRIs, considered as character strings,
are identical, then it is safe to conclude that they are equivalent.
This type of equivalence test has very low computational
cost and is in wide use in a variety of applications,
particularly in the domain of parsing and when a definitive
answer to the question of IRI equivalence is
needed that is independent of the scheme used and
can be calculated quickly and without accessing a
network. An example of such a case is XML Namespaces ().Testing strings for equivalence requires some basic precautions.
This procedure is often referred to as "bit-for-bit" or "byte-for-byte" comparison,
which is potentially misleading. Testing of strings for equality is
normally based on pairwise comparison of the characters
that make up the strings, starting from the first and
proceeding until both strings are exhausted and all
characters found to be equal, a pair of characters
compares unequal, or one of the strings is exhausted
before the other.Such character comparisons require that each pair
of characters be put in comparable encoding form. For
example, should one IRI be stored in a byte array in
UTF-8 encoding form, and the second be in a UTF-16
encoding form, bit-for-bit comparisons applied naively
will produce errors. It is better to speak of equality on
a character-for-character rather than byte-for-byte or bit-for-bit basis.
In practical terms, character-by-character comparisons should be done
codepoint-by-codepoint after conversion to a common character encoding form.
When comparing character-by-character, the comparison function MUST NOT map IRIs to
URIs, because such a mapping would create additional
spurious equivalences. It follows that IRIs SHOULD
NOT be modified when being transported if there is
any chance that this IRI might be used as an identifier.False negatives are caused by the production and
use of IRI aliases. Unnecessary aliases can be reduced,
regardless of the comparison method, by consistently
providing IRI references in an already-normalized
form (i.e., a form identical to what would be produced
after normalization is applied, as described below).
Protocols and data formats often choose to limit some
IRI comparisons to simple string comparison, based
on the theory that people and implementations will,
in their own best interest, be consistent in providing IRI
references, or at least consistent enough to negate
any efficiency that might be obtained from further
normalization.Implementations may use logic based on the definitions provided
by this specification to reduce the probability of false negatives. Such processing
is moderately higher in cost than character-for-character
string comparison. For example, an application using this approach
could reasonably consider the following two IRIs equivalent:
example://a/b/c/%7Bfoo%7D/ros&#xE9;
eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
Web user agents, such as browsers, typically apply this
type of IRI normalization when determining whether
a cached response is available. Syntax-based normalization
includes such techniques as case normalization,
character normalization, percent-encoding normalization,
and removal of dot-segments.For all IRIs, the hexadecimal digits within a percent-encoding
triplet (e.g., "%3a" versus "%3A") are case-insensitive
and therefore should be normalized to use uppercase letters
for the digits A-F.When an IRI uses components of the generic syntax, the
component syntax equivalence rules always apply;
namely, that the scheme and US-ASCII only host are case-insensitive
and therefore should be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is equivalent to
<http://www.example.com/>. Case equivalence for non-ASCII characters in IRI components that are IDNs are discussed in .
The other generic syntax components are
assumed to be case-sensitive unless specifically
defined otherwise by the scheme.Creating schemes that allow case-insensitive syntax
components containing non US-ASCII characters should
be avoided because such a case normalization may be
cultural dependant and is always a complex
operation. The only exception concerns non-ASCII host
names for which the character normalization
includes a mapping step derived from case folding.The Unicode Standard defines various equivalences
between sequences of characters for various
purposes. Unicode Standard Annex #15 defines
various Normalization Forms for these
equivalences, in particular Normalization Form C
(NFC, Canonical Decomposition, followed by Canonical
Composition) and Normalization Form KC (NFKC,
Compatibility Decomposition, followed by Canonical
Composition).Equivalence of IRIs MUST rely on the assumption that
IRIs are appropriately pre-character-normalized,
rather than applying character normalization when
comparing two IRIs. The exceptions are conversion from a non-digital form, and conversion from a non-UCS-based character encoding
to an UCS-based character encoding. In these cases, NFC or a normalizing transcoder
using NFC MUST be used for interoperability. To
avoid false negatives and problems with transcoding,
IRIs SHOULD be created using NFC. Using NFKC
may avoid even more problems, for example by choosing
half-width Latin letters instead of full-width, and
full-width Katakana instead of half-width.As an example, http://www.example.org/r&#xE9;sum&#xE9;.html
(in XML Notation) is in NFC. On the other
hand, http://www.example.org/re&#x301;sume&#x301;.html
is not in NFC. The former uses precombined e-acute
characters, the latter uses 'e' characters followed
by combining acute accents. Both usages are
defined to be canonically equivalent in .
Because it is unknown how a particular sequence of characters
is being treated with respect to character
normalization, it would be inappropriate to allow third
parties to normalize an IRI arbitrarily. This does not
contradict the recommendation that when a resource is created,
its IRI should be as character-normalized as
possible (i.e. NFC or even NFKC). This is similar to the upper-case/lower-case problems in
character-normalized as possible (i.e. NFC or even NFKC).
URIs. Some parts of a URI are case-insensitive (domain name).
For others, it is unclear whether they are case-sensitive or
case-insensitive, or something in between (e.g. case-sensitive,
but if the wrong case is used, a multiple choice selection is
provided instead of a direct negative result). The best recipe is
that the creator uses a reasonable capitalization, and when
transferring the URI, that capitalization is never changed.Various IRI schemes may allow the usage of
Internationalized Domain Names (IDN) either in
the ireg-name part or elsewhere. Character Normalization also applies to IDNs, as discussed in .The percent-encoding mechanism (Section 2.1 of )
is a frequent source of variance among
otherwise identical IRIs. In addition to the case
normalization issue noted above, some IRI producers
percent-encode octets that do not require percent-encoding,
resulting in IRIs that are equivalent to their nonencoded
counterparts. Such IRIs should be normalized by decoding
any percent-encoded octet sequence that
corresponds to an unreserved character, as described
in Section 2.3 of .For actual resolution, differences in percent-encoding
(except for the percent-encoding of reserved
characters) MUST always result in the same resource.
For example, http://example.org/~user,
http://example.org/%7euser and http://example.org/%7Euser
must resolve to the same resource.If this kind of equivalence is to be tested, the
percent-encoding of both IRIs to be compared has to be
aligned, for example by converting both IRIs to URIs
(see Section 3.1), eliminating escape differences in the
resulting URIs, and making sure that the case of the
hexadecimal characters in the percent-encoding is
always the same (preferably upper case). If the IRI
is to be passed to another application, or used further in
some other way, its original form MUST be preserved;
the conversion described here should be performed
only for the purpose of local comparison.The complete path segments "." and ".." are intended
only for use within relative references (Section 4.1 of
) and are removed as part
of the reference resolution process (Section 5.2 of ).
However, some implementations may incorrectly assume
that reference resolution is not necessary when
the reference is already an IRI, and thus fail to remove
dot-segments when they occur in non-relative paths.
IRI normalizers should remove dot-segments by applying
the remove_dot_segments algorithm to the path,
as described in Section 5.2.4 of .The syntax and semantics of IRIs vary from scheme
to scheme, as described by the defining specification
for each scheme. Implementations may use scheme-specific
rules, at further processing cost, to reduce the
probability of false negatives. For example, since the
"http" scheme makes use of an authority component,
has a default port of "80", and defines an empty path
to be equivalent to "/", the following four IRIs are
equivalent:
http://example.com
http://example.com/
http://example.com:/
http://example.com:80/In general, an IRI that uses the generic syntax for
authority with an empty path should be normalized to a
path of "/"; likewise, an explicit ":port", where the
port is empty or the default for the scheme, is equivalent to
one where the port and its ":" delimiter are elided,
and thus should be removed by scheme-based
normalization. For example, the second IRI above is
the normal form for the "http" scheme.Another case where normalization varies by scheme
is in the handling of an empty authority component or
empty host subcomponent. For many scheme specifications,
an empty authority or host is considered an
error; for others, it is considered equivalent to "localhost"
or the end-user's host. When a scheme defines a
default for authority and an IRI reference to that default
is desired, the reference should be normalized to an
empty authority for the sake of uniformity, brevity,
and internationalization. If, however, either the userinfo or
port subcomponent is non-empty, then the host should
be given explicitly even if it matches the default.Normalization should not remove delimiters when their
associated component is empty unless licensed to do
so by the scheme specification. For example, the IRI
"http://example.com/?" cannot be assumed to be
equivalent to any of the examples above. Likewise, the
presence or absence of delimiters within a userinfo
subcomponent is usually significant to its interpretation.
The fragment component is not subject to any
scheme-based normalization; thus, two IRIs that differ
only by the suffix "#" are considered different
regardless of the scheme.Some IRI schemes may allow the usage of
Internationalized Domain Names (IDN) either in
their ireg-name part or elsewhere. When in use in IRIs,
those names SHOULD be validated using the
ToASCII operation defined in , with the flags
"UseSTD3ASCIIRules" and "AllowUnassigned". An
IRI containing an invalid IDN cannot successfully be resolved.
Validated IDN components of IRIs SHOULD
be character normalized using the Nameprep process ;
however, for legibility purposes, they
SHOULD NOT be converted into ASCII Compatible Encoding (ACE).Scheme-based normalization may also consider IDN components and their conversions to punycode as equivalent. As an example, http://r&#xE9;sum&#xE9;.example.org may be considered equivalent to
http://xn--rsum-bpad.example.orgOther scheme-specific normalizations are possible.Web spiders, for which substantial effort to reduce the
incidence of false negatives is often cost-effective,
are observed to implement even more aggressive techniques
in IRI comparison. For example, if they
observe that an IRI such as
http://example.com/dataredirects to an IRI differing only in the trailing slash
http://example.com/data/they will likely regard the two as equivalent in the future.
This kind of technique is only appropriate when
equivalence is clearly indicated by both the result of
accessing the resources and the common conventions
of their scheme's dereference algorithm (in this case,
use of redirection by HTTP origin servers to avoid
problems with relative references).This section discusses limitations on characters and
character sequences usable for IRIs beyond those given in
and . The considerations in this
section are relevant when creating IRIs and when converting from URIs
to IRIs.The repertoire of characters allowed
in each IRI component is limited by the definition of that component.
For example, the definition of the scheme component does not allow
characters beyond US-ASCII.
(Note: In accordance with URI practice, generic IRI
software cannot and should not check for such limitations.)The UCS contains many areas of characters for which there are
strong visual look-alikes. Because of the likelihood of
transcription errors, these also should be avoided. This includes
the full-width equivalents of Latin characters, half-width
Katakana characters for Japanese, and many others. This also
includes many look-alikes of "space", "delims", and "unwise",
characters excluded in .Additional information is available from .
is written in the context of running text rather
than in the context of identifiers. Nevertheless, it discusses many of the
categories of characters not appropriate for IRIs.Although an IRI is defined as a sequence of characters,
software interfaces for URIs typically function on sequences of
octets or other kinds of code units. Thus, software interfaces
and protocols MUST define which character encoding is used.Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs per ,
when transferring from IRI-capable to URI-only components. Such
a mapping SHOULD be applied as late as possible. It SHOULD NOT be
applied between components that are known to be able to handle IRIs.Document formats that transport URIs may need to be upgraded to allow
the transport of IRIs. In those cases where the document as a whole
has a native character encoding, IRIs MUST also be encoded in this
character encoding, and converted accordingly by a parser or interpreter.
IRI characters that are not expressible in the native character encoding SHOULD
be escaped using the escaping conventions of the document format
if such conventions are available. Alternatively, they MAY be percent-encoded
according to . For example, in HTML or
XML, numeric character references SHOULD be used. If a document
as a whole has a native character encoding, and that character encoding
is not UTF-8, then IRIs MUST NOT be placed into the document in the
UTF-8 character encoding.Note: Some formats already accommodate IRIs, although they use
different terminology. HTML 4.0 defines the conversion from
IRIs to URIs as error-avoiding behavior. XML 1.0 , XLink
, and XML Schema and specifications
based upon them allow IRIs. Also, it is expected that all relevant new W3C
formats and protocols will be required to handle IRIs .This section discusses details and gives examples
for point c) in . In order to be able to
use IRIs, the URI corresponding to the IRI in question has to
encode original characters into octets using UTF-8.
This can be specified for all URIs of a URI scheme, or can apply to individual
URIs for schemes that do not specify how to encode original characters.
It can apply to the whole URI, or only some part. For background information
on encoding characters into URIs, see also Section 2.5 of .For new URI schemes, using UTF-8 is recommended in .
Examples where UTF-8 is already used are the URN syntax ,
IMAP URLs , and POP URLs .
On the other hand, because the HTTP URL scheme does not specify how to encode
original characters, only some HTTP URLs can have corresponding but different IRIs.For example, for a document with a URI of
http://www.example.org/r%C3%A9sum%C3%A9.html,
it is possible to construct a corresponding IRI (in XML notation,
see ):
http://www.example.org/r&#xE9;sum&#xE9;.html (&#xE9; stands for the
e-acute character, and %C3%A9 is the UTF-8 encoded and percent-encoded representation
of that character). On the other hand, for a document with
a URI of http://www.example.org/r%E9sum%E9.html, the percent-encoding
octets cannot be converted to actual characters in an IRI,
because the percent-encoding is not based on UTF-8.This means that for most URI schemes, there is no need to upgrade their
scheme definition in order for them to work with IRIs.
The main case where
upgrading a scheme definition makes sense is when a scheme definition,
or a particular component of a scheme,
is strictly limited to the use of US-ASCII characters
with no provision to include non-ASCII characters/octets via percent-encoding,
or if a scheme definition currently uses
highly scheme-specific provisions for the encoding of non-ASCII characters.
An example of such a
scheme might be the mailto: scheme .This specification does not upgrade any scheme specifications in any
way, this has to be done separately. Also, it should be noted that there
is no such thing as an "IRI scheme"; all IRIs use URI schemes, and all
URI schemes can be used with IRIs, even though in some cases only
by using URIs directly as IRIs, without any conversion.URI schemes can impose restrictions on the syntax of
scheme-specific URIs, ie. URIs that are admissable under the
generic URI syntax [RFCYYYY] may not be admissable due to
narrower syntactic constraints imposed by a URI scheme
specification. URI scheme definitions cannot broaden the
syntactic restrictions of the generic URI syntax, otherwise
it would be possible to generate URIs that satisfied the
scheme specific syntactic constraints without satisfying the
syntactic constraints of the generic URI syntax. However,
additional syntactic constraints imposed by URI scheme
specifications are applicable to IRI since the
corresponding URI resulting from the mapping defined in
MUST be a valid URI under the syntactic
restrictions of generic URI syntax and any narrower
restrictions imposed by the corresponding URI scheme
specification.The requirement for the use of UTF-8 applies to all parts of a URI
(with the potential exception of the ireg-name part,
see ). However, it is possible that
the capability of IRIs to represent a wide range of characters directly
is used just in some parts of the IRI (or IRI reference). The other parts
of the IRI may only contain US-ASCII characters, or they may not be based
on UTF-8. They may be based on another character encoding, or they may
directly encode raw binary data (see also ).For example, it is possible to have a URI reference of
http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9,
where the document name is encoded in iso-8859-1 based on server
settings, but the fragment identifier is encoded in UTF-8 according
to . The IRI corresponding to the above
URI would be (in XML notation)
http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9;.Similar considerations apply to query parts. The functionality
of IRIs (namely to be able to include non-ASCII characters) can
only be used if the query part is encoded in UTF-8.Processing of relative IRI references against a base is handled
straightforwardly; the algorithms of can
be applied directly, treating the characters additionally allowed
in IRI references in the same way as unreserved characters in URI
references.This informative section provides guidelines for supporting IRIs
in the same software components and operations that currently process
URIs: software interfaces that handle URIs, software that allows users
to enter URIs, software that creates or generates URIs, software that displays
URIs, formats and protocols that transport URIs, and software that
interprets URIs. These may all require more or less modification before
functioning properly with IRIs. The considerations in this section
also apply to URI references and IRI references.Software interfaces that handle URIs, such as URI-handling APIs and
protocols transferring URIs, need interfaces and protocol elements
that are designed to carry IRIs.In case the current handling in an API or protocol is based on
US-ASCII, UTF-8 is recommended as the character encoding for IRIs,
because this is compatible with US-ASCII, is in accordance with the
recommendations of , and makes it easy to
convert to URIs where necessary. In any case, the API or protocol
definition must clearly define the character encoding to be used.The transfer from URI-only to IRI-capable components requires no
mapping, although the conversion described in above may be
performed. It is preferable not to perform this inverse conversion
when there is a chance that this cannot be done correctly.There are components that allow users to enter URIs into the system,
for example by typing or dictation. This software must be updated to allow
for IRI entry.A person viewing a visual representation of an IRI (as a sequence of
glyphs, in some order, in some visual display) or hearing an IRI,
will use a entry method for characters in the user's language to input
the IRI. Depending on the script and the input method used, this may
be a more or less complicated process.The process of IRI entry must assure, as far as possible, that the
restrictions defined in are met. This may be done by
choosing appropriate input methods or variants/settings thereof, by
appropriately converting the characters being input, by eliminating
characters that cannot be converted, and/or by issuing a warning or
error message to the user.As an example of variant settings, input method editors for East
Asian Languages usually allow the input of Latin letters and related
characters in full-width or half-width versions. For IRI input, the
input method editor should be set so that it produces half-width
Latin letters and punctuation, and full-width Katakana.An input field primarily or only used for the input of URIs/IRIs
may allow the user to view an IRI as mapped to a URI. Places
where the input of IRIs is frequent may provide the possibility for
viewing an IRI as mapped to a URI. This will help users when some
of the software they use does not yet accept IRIs.An IRI input component that interfaces to components that handle URIs,
but not IRIs, must map the IRI to a URI before passing it to such a
component.For the input of IRIs with right-to-left characters, please see
.Many applications, in particular many mail user agents, try to detect
URIs appearing in plain text. For this, they use some heuristics based
on URI syntax. They then allow the user to click on such
URIs and retrieve the corresponding resource in an appropriate (usually
scheme-dependent) application.Such applications have to be upgraded to use the IRI syntax rather
than the URI syntax as a base for heuristics. In particular, a non-ASCII
character should not be taken as the indication of the end of an IRI.
Such applications also have to make sure that they correctly convert the
detected IRI from the character encoding of the document or application where
the IRI appears to the character encoding used by the system-wide IRI invocation
mechanism, or to a URI (according to ) if
the system-wide invocation mechanism only accepts URIs.The clipboard is another frequently used way to transfer URIs and
IRIs from one application to another. On most platforms, the clipboard
is able to store and transfer text in many languages and scripts.
Correctly used, the clipboard transfers characters, not bytes,
which will do the right thing with IRIs.Systems that offer resources through the Internet, where those
resources have logical names, sometimes automatically generate URIs
for the resources they offer. For example, some HTTP servers can
generate a directory listing for a file directory,
and then respond to the generated URIs with the files.Many legacy character encodings are in use in various file systems.
Many currently deployed systems do not transform the local character
representation of the underlying system before generating URIs.For maximum interoperability, systems that generate resource
identifiers should do the appropriate transformations. For example,
if a file system contains a file named r&#xE9;sum&#xE9;.html,
a server should expose this as r%C3%A9sum%C3%A9.html in a URI, which
allows to use r&#xE9;sum&#xE9;.html in an IRI,
even if the file name locally is kept in a character
encoding other than UTF-8.
This recommendation in particular applies to HTTP servers. For FTP
servers, similar considerations apply, see in particular .In some cases, resource owners and publishers have control over the
IRIs used to identify their resources. Such control is mostly
executed by controlling the resource names, such as file names,
directly.In such cases, it is recommended to avoid choosing IRIs that are
easily confused. For example, for US-ASCII, the lower-case ell "l" is
easily confused with the digit one "1", and the upper-case oh "O" is
easily confused with the digit zero "0". Publishers should avoid
confusing users with "br0ken" or "1ame" identifiers.Outside of the US-ASCII repertoire, there are many more opportunities for
confusion; a complete set of guidelines is too lengthy to include
here. As long as names are limited to characters from a single script,
native writers of a given script or language will know best when
ambiguities can appear, and how they can be avoided. What may look
ambiguous to a stranger may be completely obvious to the average
native user. On the other hand, in some cases, the UCS contains
variants for compatibility reasons, for example for typographic purposes.
These should be avoided wherever possible. Although there may be exceptions,
in general newly created resource names should be in NFKC
(which means that they are also in NFC).As an example, the UCS contains the 'fi' ligature at U+FB01
for compatibility reasons.
Wherever possible, IRIs should use the two letters 'f' and 'i' rather
than the 'fi' ligature. An example where the latter may be used is
in the query part of an IRI for an explicit search for a word written
containing the 'fi' ligature.In certain cases, there is a chance that characters from different
scripts look the same. The best known example is the Latin 'A', the
Greek 'Alpha', and the Cyrillic 'A'. To avoid such cases, only IRIs
should be created where all the characters in a single component are
used together in a given language. This usually means that all these
characters will be from the same script, but there are languages that
mix characters from different scripts (such as Japanese).
This is similar to the heuristics used to distinguish between
letters and numbers in the examples above. Also, for Latin, Greek,
and Cyrillic, using lower-case letters results in fewer ambiguities
than using upper-case letters.
In situations where the rendering software is not expected to display
non-ASCII parts of the IRI correctly using the available layout and font
resources, these parts should be percent-encoded before being displayed.For display of Bidi IRIs, please see .Software that interprets IRIs as the names of local resources should
accept IRIs in multiple forms, and convert and match them with the
appropriate local resource names.First, multiple representations include both IRIs in the native
character encoding of the protocol and also their URI counterparts.Second, it may include URIs constructed based on other character
encodings than UTF-8. Such URIs may be produced by user agents that do
not conform to this specification and use legacy character encodings to
convert non-ASCII characters to URIs. Whether this is necessary and what
character encodings to cover, depends on a number of factors, such as
the legacy character encodings used locally and the distribution of
various versions of user agents. For example, software for Japanese
may accept URIs in Shift_JIS and/or EUC-JP in addition to UTF-8.Third, it may include additional mappings to be more user-friendly and
robust against transmission errors. These would be similar to how
currently some servers treat URIs as case-insensitive, or perform
additional matching to account for spelling errors. For characters
beyond the US-ASCII repertoire, this may for example include ignoring the
accents on received IRIs or resource names where appropriate. Please
note that such mappings, including case mappings, are
language-dependent.It can be difficult to unambiguously identify a resource if too
many mappings are taken into consideration. However, percent-encoded
and not percent-encoded parts of IRIs can always clearly be distinguished.
Also, the regularity of UTF-8 (see ) makes the
potential for collisions lower than it may seem at first sight.Where this recommendation places further constraints on software
for which many instances are already deployed, it is important to
introduce upgrades carefully, and to be aware of the various
interdependencies.If IRIs cannot be interpreted correctly, they should not be created,
generated, or transported. This suggests that upgrading URI interpreting
software to accept IRIs should have highest priority.On the other hand, a single IRI is interpreted only by a single or
very few interpreters that are known in advance, while it may be
entered and transported very widely.Therefore, IRIs benefit most from a broad upgrade of software to be
able to enter and transport IRIs, but before publishing any
individual IRI, care should be taken to upgrade the corresponding
interpreting software in order to cover the forms expected to be
received by various versions of entry and transport software.The upgrade of generating software to generate IRIs instead of using a
local character encoding should happen only after the service is upgraded
to accept IRIs. Similarly, IRIs should only be generated when the service
accepts IRIs and the intervening infrastructure and protocol is known
to transport them safely.Software converting from URIs to IRIs for display should be upgraded
only after upgraded entry software has been widely deployed to the
population that will see the displayed result.It is often possible to reduce the effort and dependencies for upgrading
to IRIs by using UTF-8 rather than another character encoding where there is
a free choice of character encodings. For example, when setting up a
new file-based Web server, using UTF-8 as the character encoding for file
names will make the transition to IRIs easier. Likewise, when setting up a
new Web form using UTF-8 as the character encoding of the form page, the
returned query URIs will use UTF-8 as the character encoding (unless the user,
for whatever reason, changes the character encoding) and will therefore be
compatible with IRIs.These recommendations, when taken together, will allow for the
extension from URIs to IRIs in order to handle characters other than
US-ASCII while minimizing interoperability problems. For considerations
regarding the upgrade of URI scheme definitions, please see .The security considerations discussed in
also apply to IRIs. In addition, the following issues require
particular care for IRIs.Incorrect encoding or decoding can lead to security problems.
In particular, some UTF-8 decoders do not check against overlong
byte sequences. As an example, a '/' is encoded with the byte 0x2F
both in UTF-8 and in US-ASCII, but some UTF-8 decoders also wrongly
interpret the sequence 0xC0 0xAF as a '/'. A sequence such as '%C0%AF..'
may pass some security tests and then be interpreted
as '/..' in a path if UTF-8 decoders are fault-tolerant, if conversion
and checking are not done in the right order, and/or if reserved
characters and unreserved characters are not clearly distinguished.There are various ways in which "spoofing" can occur with IRIs.
"Spoofing" means that somebody may add a resource name that looks the
same or similar to the user, but points to a different resource.
The added resource may pretend to be the real resource by looking
very similar, but may contain all kinds of changes that may be
difficult to spot and can cause all kinds of problems.
Most spoofing possibilities for IRIs are extensions of those for URIs.Spoofing can occur for various reasons. A first reason is
that normalization expectations of a user or actual normalization
when entering an IRI, or when transcoding an IRI from a legacy character
encoding, do not match the normalization used on the
server side. Conceptually, this is no different from the problems
surrounding the use of case-insensitive web servers. For example,
a popular web page with a mixed case name (http://big.example.com/PopularPage.html)
might be "spoofed" by someone who is able to create http://big.example.com/popularpage.html.
However, the use of unnormalized character sequences, and of additional
mappings for user convenience, may increase the chance for spoofing.
Protocols and servers that allow the creation of resources with
names that are not normalized are particularly vulnerable to such
attacks. This is an inherent
security problem of the relevant protocol, server, or resource,
and not specific to IRIs, but mentioned here for completeness.Spoofing can occur in various IRI components, such as the
domain name part or a path part. For considerations specific
to the domain name part, see .
For the path part, administrators of sites which allow independent
users to create resources in the same subarea may need to be careful
to check for spoofing.Spoofing can occur because in the UCS, there are many characters that
look very similar. Details are discussed in .
Again, this is very similar to spoofing possibilities on US-ASCII,
e.g. using 'br0ken' or '1ame' URIs.Spoofing can occur when URIs with percent-encodings based on various
character encodings are accepted to deal with older user agents. In some
cases, in particular for Latin-based resource names, this is usually easy to
detect because UTF-8-encoded names, when interpreted and viewed as
legacy character encodings, produce mostly garbage. In other cases, when
concurrently used character encodings have a similar structure, but there
are no characters that have exactly the same encoding, detection is more
difficult.Spoofing can occur with bidirectional IRIs, if the restrictions
in are not followed. The same visual
representation may be interpreted as different logical representations,
and vice versa. It is also very important that a correct Unicode bidirectional
implementation is used.This document has no actions for IANA.We would like to thank Larry Masinter for his work as
coauthor of many earlier versions of this document
(draft-masinter-url-i18n-xx).The discussion on the issue addressed here has started a long time
ago. There was a thread in the HTML working
group in August 1995 (under the topic of "Globalizing URIs") and in the
www-international mailing list in July 1996 (under the topic of
"Internationalization and URLs"), and ad-hoc meetings at the Unicode
conferences in September 1995 and September 1997.Many thanks go to Francois Yergeau, Matitiahu Allouche,
Roy Fielding, Tim Berners-Lee, Mark Davis,
M.T. Carrasco Benitez, James Clark, Tim Bray, Chris Wendt, Yaron Goland,
Andrea Vine, Misha Wolf, Leslie Daigle, Ted Hardie, Bill Fenner, Margaret Wasserman,
Russ Housley, Makoto MURATA, Steven Atkin,
Ryan Stansifer, Tex Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Ian Jacobs,
Adam Costello, Dan Oscarson, Elliotte Rusty Harold, Mike J. Brown,
Roy Badami, Jonathan Rosenne, Asmus Freytag, Simon Josefsson, Carlos Viegas Damasio,
Chris Haynes, Walter Underwood, and many others for help with
understanding the issues and possible solutions, and getting the details right.This document is a product of the Internationalization Working
Group (I18N WG) of the World Wide Web Consortium (W3C).
Thanks to the members of the W3C
I18N Working Group and Interest Group for their contributions and their
work on . Thanks also go
to the members of many other W3C Working Groups for adopting IRIs, and to
the members of the Montreal IAB Workshop on Internationalization and
Localization for their review.Coded Character Set -- 7-bit American Standard Code for Information
InterchangeAmerican National Standards InstituteISO/IEC 10646:2003: Information Technology -
Universal Multiple-Octet Coded Character Set (UCS)International Organization for StandardizationKey words for use in RFCs to Indicate Requirement LevelsGeneral
keywordAugmented BNF for Syntax Specifications: ABNFInternationalizing Domain Names in Applications (IDNA)Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)UTF-8, a transformation format of ISO 10646Uniform Resource Identifier (URI): Generic Syntax
(Note to the RFC Editor: Please update this reference with the RFC resulting from
draft-fielding-uri-rfc2396bis-xx.txt, and remove this Note)Please update this reference with the successor of RFC 2396, currently
draft-fielding-uri-rfc2396bis-07.txtThe Unicode Standard, Version 4.0.1, defined by: The Unicode Standard,
Version 4.0 (Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
as amended by Unicode 4.0.1 (http://www.unicode.org/versions/Unicode4.0.1/)The Unicode ConsortiumThe Bidirectional AlgorithmUnicode Normalization FormsExamples of bidirectional IRIsCharacter Model for the World Wide WebThe Properties and Promises of UTF-8URI Model ConsequencesHTML 4.01 SpecificationMultipurpose Internet Mail Extensions
(MIME) Part One:
Format of Internet Message BodiesThe Report of the IAB Character Set Workshop held 29 February - 1 March, 1996Applications
Internet Architecture Boardcharacter encodingmultipurpose internet mail extensionsworkshopURN SyntaxApplications
URNuniform resourceIMAP URL SchemeApplications
IMAPinternet message access protocoluniform resourceIETF Policy on Character Sets and LanguagesApplications
Internet Engineering Task Forcecharacter encodingThe mailto URL schemePOP URL SchemeApplications
POPpost office protocoluniform resourceUniform Resource Identifiers (URI): Generic SyntaxApplications
uniform resourceURIThe "data" URL schemeApplications
URLuniform resourceHypertext Transfer Protocol -- HTTP/1.1Internationalization of the File Transfer ProtocolGuidelines for new URL SchemesUnicode in XML and other Markup LanguagesXML Linking Language (XLink) Version 1.0Extensible Markup Language (XML) 1.0 (Third Edition)Namespaces in XMLXML Schema Part 2: DatatypesXPointer FrameworkThis section shortly summarizes major design alternatives
and the reasons for why they were not chosen.Introducing new schemes (for example httpi:, ftpi:,...) or a
new metascheme (e.g. i:, leading to URI/IRI prefixes such as
i:http:, i:ftp:,...) was proposed to make IRI-to-URI conversion
scheme-dependent or to distinguish between percent-encodings
resulting from IRI-to-URI conversion and percent-encodings from
legacy character encodings.New schemes are not needed to distinguish URIs from true IRIs (i.e.
IRIs that contain non-ASCII characters). The benefit of being able
to detect the origin of percent-encodings is marginal, because UTF-8
can be detected with very high reliability. Deploying new schemes is
extremely hard, so not requiring new schemes for IRIs makes
deployment of IRIs vastly easier. Making conversion scheme-dependent
is highly inadvisable, and would be encouraged by separate schemes for IRIs.
Using an uniform convention for conversion from IRIs to URIs makes
IRI implementation orthogonal to the introduction of actual new
schemes.At an early stage, UTF-7 was considered as an alternative to
UTF-8 when converting IRIs to URIs. UTF-7 would not have needed
percent-encoding, and would in most cases have been shorter than
percent-encoded UTF-8.Using UTF-8 avoids a double layering and overloading of the use of
the "+" character. UTF-8 is fully compatible with US-ASCII, and has
therefore been recommended by the IETF, and is being used widely,
while UTF-7 has never been used much and is now clearly being
discouraged. Requiring implementations to convert from UTF-8
to UTF-7 and back would be an additional implementation burden.Instead of using the existing percent-encoding convention
of URIs, which is based on octets, the idea was to create a new
encoding convention, for example to use '%u' to introduce
UCS code points.Using the existing octet-based percent-encoding mechanism
does not need an upgrade of the URI syntax, and does not
need corresponding server upgrades.Some proposals suggested indicating the character encodings used
in an URI or IRI with some new syntactic convention in the URI itself,
similar to the 'charset' parameter for emails and Web pages.
As an example, the label in square brackets in
http://www.example.org/ros[iso-8859-1]&#xE9; indicated that
the following &#xE9; had to be interpreted as iso-8859-1.Using UTF-8 only does not need an upgrade to the URI syntax.
It avoids potentially multiple labels that have to be copied correctly
in all cases, even on the side of a bus or on a napkin, leading to
usability problems to the extent of being prohibitively annoying.
Using UTF-8 only also reduces transcoding errors and confusions.